1. Field of the Invention
The prsent invention relates generally to circuits for starting single-phase, alternating current induction electric motors and, more particularly, but not by way of limitation, to circuits which momentarily energize a field winding in such motors.
2. Brief Discussion of the Prior Art
The operation of an induction electric motor generally requires that electrically induced magnetic fields in the motor vary in direction with time. Where multiple phase electric power is available, this requirement is easily met by arranging field windings, each energized at a different phase, about the rotor of the motor such that the magnetic fields produced by the individual field, or main, windings add vectorially to produce a rotating magnetic field which the rotor follows. Single-phase motors, on the other hand, generally require some other scheme, at least during a short period following start-up, to cause windings in the motor to produce magnetic fields having a rotational character. Once started, the rotating rotor will itself give rise to induced magnetic fields such that a two-pole field winding is capable of maintaining the rotation of the rotor and transmitting torque thereto so that the motor can do useful work. Among the various schemes for starting a motor is the inclusion of a starting winding which is energized for only a short period of time following start-up to permit the motor to be brought up to operating speed and which is thereafter de-energized. Both the commercially available split-phase and capacitor start motors use such a scheme.
In the past, it has been standard practice to use a centrifugal switch to energize the starting winding for the short period following start-up. Such switches are connected in series with the starting windings of motors and are of the normally closed type. They are mounted on the rotors of the motors and their contacts are arranged so that a centrifugal force on the mounting for one contact will be thrown radially outwardly, away from the other contact, when the rotor upon which such a switch is mounted reaches a preselected speed of rotation.
While centrifugal switches have thus provided a simple means for momentarily energizing the starting winding of a motor, their use has not been without problems. These switches are fairly expensive and they have only a limited lifetime. That is, the switch must be replaced after a number of make and break operations. This lifetime limitation can be an important economic factor where a motor is used in an application in which the motor is repetitively operated at frequent intervals.